1. Field of the Invention
The present invention relates to an optical sensor interrogation system, and more particularly, to an optical sensor interrogation system which may monitor absolute values of physical quantities such as temperature, pressure, and strain in real-time at high speed by using a wavelength swept laser.
2. Description of the Related Art
In general, fiber Bragg grating (FBG) sensors irradiate ultraviolet rays to the core of optical fiber on which specific formed pattern (grating) is formed and induce a change of a refractive index. In this case, the FBG sensor selectively reflects light with a specific wavelength (Bragg wavelength) and a wavelength of light reflected by a pattern interval changes.
Accordingly, when a pattern interval changes due to external stress, a wavelength tune in reflected light may be observed and a change of the pattern interval, that is, strain, may be measured. In addition, the FBG sensor is not affected by an electromagnetic wave and the optical fiber including silica SiO2is an insulator. Also, the FBG sensor is small and light-weighted and thus may be installed and inserted without affecting functions of subjects to be measured. In addition, the FBG sensor may transmit optical signals to a distance without loss and thus facilitate telemetering. Moreover, a large number of FBG sensors may be installed at various places and used to measure at the same time so that multiplex measurement is available.
A method of realizing a general optical sensor interrogation system includes measuring a fiber grating wavelength by using a light source having broad line width and a wavelength tunable filter.
The other method of realizing a general optical sensor interrogation system includes using a wavelength swept laser as a light source. In this method, a wavelength tunable filter is used to tune a wavelength of the wavelength swept laser over a wavelength range of fiber gratings according to time, and the time of a peak in a fiber grating reflection signal is measured by using the specific relation between the time and the laser wavelength so that the wavelengths of the fiber gratings may be measured.
The general optical sensor interrogation system may be essential to liquefied natural gas (LNG) shipping which stores and transports LNG at a super low temperature. Since a tank storing LNG may be exploded even by a small crack, a real-time analysis which is performed at high speed for precisely measuring fluid flowing pressure and temperature is important.
In the optical sensor interrogation system according to the first method above, a peak of a reflected signal is low and the line width is great so that precise measurement is difficult.
For the real-time analysis which is performed at high speed for precisely measuring the strain of a fluid, wavelength tunable speed of 10 kHz or above is needed. In the optical sensor interrogation system using the wavelength swept laser according to the second method above, a peak of a reflected signal is high and the line width is narrow. However, regular optical gain acquisition response time in a resonator may not conform to an increase of wavelength tunable time. Thus, speed maintains few kHz or below.